If it were me, I wouldn't have any fruit trees in my pasture. I think of it as the equivalent of leaving the door open to the grain storage. I know that my horse would not stop eating, especially if it were apples. I have a friend who kept an older horse in a pasture with a couple of apple trees. They went out and had to pick up the fruit daily to keep the horse from over eating, they went on vacation for a few days and the horse colicked. The neighbors were watching the property, but didn't know to pick up the apples.
The other problem with fruit trees is they can be very messy. You have to pick up the fallen fruit frequently or you can have a huge problem with insects and rodents which is also not great for horses.
If you love the idea of an orchard and want to keep some of the trees, you can always fence off a couple of the trees in the pasture. If maintained, it can make a lovely addition to your home, and your horses would love a fresh apple from time to time.